A. Catana

Screw dislocation mediated growth of sputtered and laser-ablated YBa2Cu3O7−δ films

By imaging the as-grown surfaces of sputtered and laserablated YBa2Cu3O7−δ films with scanning tunneling microscopy (STM), we have directly observed spiral-shaped growth terraces which emanate from screw dislocations. The density of screw dislocations was observed to decrease with increasing growth temperature and substrate misorientation. The surface structures observed by STM together with cross-sectional transmission electron microscope (TEM) images provide insights into the mechanisms of crystal growth operative during the formation of YBa2Cu3O7−δ films grown using these two widespread techniques.

Sr2RuO4: A metallic substrate for the epitaxial growth of YBa2Cu3O7−δ

Single crystals of Sr2RuO4 were grown by the floating zone melting technique. The crystals have the K2NiF4 structure and display a metallic resistivity behavior in the a‐b‐plane between 300 and 4.2 K (ρab≂10−4 Ω cm at 300 K). The in‐plane lattice mismatch between YBa2Cu3O7−δ (001) and Sr2RuO4 (001) is smaller than 1.3%, better than that to SrTiO3 {100}. Epitaxial films of YBa2Cu3O7−δ with Tc(R=0) as high as 86 K have been grown on Sr2RuO4 crystals. The epitaxial growth of YBa2Cu3O7−δ on Sr2RuO4 was revealed by four‐circle x‐ray diffraction as well as by transmission electron microscopy.

Identification of epitaxial Y2O3 inclusions in sputtered YBa2Cu3O7 films: Impact on film growth

Y2O3 inclusions with typical sizes between 100 and 300 nm3, densely distributed (1016 cm−3) in sputtered YBa2Cu3O7 (YBCO) films on SrTiO3 substrates, have been identified by high‐resolution electron microscopy. The precipitates exhibit either cuboid or needlelike shapes and grow epitaxially within and on top of YBCO. The dominant orientation relationship corresponds to a situation where the two‐dimensional lattices are nearly coincident in the interfacial (001) plane. These precipitates may contribute to the generation of screw and edge dislocations. In addition, they provide a large number of potential pinning sites for magnetic flux lines, which may contribute to the observed high critical current densities.

Block‐by‐block deposition: A new growth method for complex oxide thin films

An alternative growth method for high‐Tc oxide thin films employing molecular beam deposition is proposed. Instead of an uncontrolled local nucleation followed by lateral growth and island coalescence, the new method provides substrate coverage by nonreacting constituents before nucleation is initiated, a controlled reaction path, and reduced lateral growth. DyBa2Cu3O6+δ films without precipitates, with a surface roughness of ±1 unit cell and showing finite size oscillations in the x‐ray diffraction spectrum, have been prepared. This method reveals that diffusion dominates the growth process at high substrate temperatures (≂700 °C).

CORRELATION BETWEEN JC AND SCREW DISLOCATION DENSITY IN SPUTTERED YBA2CU3O7-DELTA FILMS

Electric transport properties of sputtered YBa2Cu3O7−δ films were studied as a function of screw dislocation density, ranging from 5·107 cm−2 to 1.3·109 cm−2 as determined at the film surface. A correlation was found between the number of screw dislocations and the critical current density (Jc). Films with higher screw dislocation densities have higher critical current densities and a slower drop ofJc as a function of applied magnetic fieldH.

Surface outgrowths on sputtered YBa2Cu3O7−x films: A combined atomic force microscopy and transmission electron microscopy study

We have structurally and morphologically characterized the surface of sputtered YBa2Cu3O7−x films on (001) SrTiO3 using atomic force microscopy and transmission electron microscopy. Atomic force microscopy reveals three types of outgrowths with different shapes and heights between 2 and 200 nm: type I exhibits cubic habit, type II tabular habit, and type III is an agglomerate of no particular shape. Some of the type‐III outgrowths are located at the center of growth spirals where the screw dislocation intersects the film surface, suggesting that in YBa2Cu3O7−x films these defects promote the occurrence of one another. Using high‐resolution electron microscopy and electron diffraction the surface outgrowths have been identified as follows: type I is Y2O3, type II Y2O3 and CuYO2, and type III YBa2Cu3O7−x, CuO, and Y2O3. In contrast to types‐I and ‐II outgrowths which are both epitaxially related to the surrounding YBa2Cu3O7−x, the large type‐III agglomerates consist of epitaxial and nonepitaxial grains. As ...

a/c YBa2Cu3O7 boundaries: Preferential sites for the nucleation of epitaxial Y2O3 precipitates

Y2O3 precipitates with typical sizes between 70 and 300 nm2 have been identified by high‐resolution electron microscopy and image calculations in mixed a‐ and c‐axis oriented YBa2Cu3O7 sputtered films. The precipitates are densely distributed (1015 cm−3), have tabular shape and grow epitaxially at boundaries between a‐ and c‐axis oriented grains, with the (001) Y2O3 plane parallel to the a, b plane of the a‐axis oriented grain and the (110) Y2O3 plane parallel to the a,b plane of the c‐axis oriented grain. Their largest interfacial facet lies parallel to the a,b plane of the a‐axis oriented regions. Lattice‐matching arguments show that energetically this situation is the most favorable one, which explains the nucleation of precipitates at a/c boundaries.

High-T c Thin Films. Growth Modes — Structure — Applications

Using microscopic investigations, a basic understanding of the growth mode of high-T c films has been obtained in the past couple of years. In this presentation a brief overview of the growth mechanism of Yu3δ films will be given, and the morphology of the films as well as their potential applications will be addressed.